After the administration of acetylaminofluorene (AAF), both Ito cells and oval cells proliferate. Staining with hematoxylin and eosin did not reveal their actual numbers due to the similarities in the shape of their nuclei. However, when specific antibodies for oval cells (OV-6) and Ito cells (desmin) were used, the proliferation of Ito cells coincided with the oval cells. No prominent expression of the transforming growth factor alpha (TGFalpha) above the level observed in the hepatocytes was detected in the areas of oval cell and Ito cell proliferation. However, areas of proliferating basophilic hepatocytes had increased levels of TGFalpha transcripts. Ligation of the common bile duct leads to the proliferation of bile duct cells, which proved negative for alpha-fetoprotein (AFP) and albumin. Northern hybridization using poly-RNA from the ligated rat livers revealed a significant expression of transforming growth factor beta1 (TGF-beta1). The thick layer of desmin positive cells around the bile ducts may play an important role in preventing the invasion of the liver parenchyma by proliferating bile ducts. Recent studies both in vivo and in vitro have demonstrated a modulation of gene expression by retinoids. We have used the uninitiated Solt-Farber protocol in an effort to elucidate the role of vitamin A in the differentiation of putative liver stem cells. Our preliminary data revealed a delayed differentiation of oval cells into hepatocytes and a metaplastic differentiation into intestinal cells in animals with vitamin A supplemented diet in contrast to the diet that was devoid of vitamin A.